The present invention relates generally to electrolytic cells. More specifically, the present invention relates to the improved electrode frames and electrodes that may be employed in electrolytic cells when the electrodes are produced by the metallurgical bonding technique of diffusion bonding and the electrode frames are formed from elastomeric materials.
Chlorine and caustic, products of the electrolytic process, are basic chemicals which have become large volume commodities in the industrialized world today. The overwhelming amounts of these chemicals are produced electrolytically from aqueous solutions of alkali metal chlorides. Cells which have traditionally produced these chemicals have come to be known as chloralkali cells. The chloralkali cells today are generally of two principal types, the deposited asbestos diaphragm-type electrolytic cell or the flowing mercury cathode-type.
Comparatively recent technological advances such as the development of dimensionally stable anodes and various coating compositions, have permitted the gap between electrodes to be substantially decreased or eliminated entirely. This has dramatically increased the energy efficiency during the operation of these energy-intensive units.
The development of a hydraulically impermeable, ion selective membrane has promoted the advent of filter press membrane chloralkali cells which produce a relatively uncontaminated caustic product. This higher purity product obviates the need for caustic purification and decreases the need for concentration processing. The use of a hydraulically impermeable planar membrane has been most common in bipolar filter press membrane electrolytic cells. However, continual advances have been made in the development of monopolar filter press membrane cells. A hydraulically impermeable membrane also has found utility in electrolytic cells other than chloralkali electrolytic cells.
The use of a hydraulically impermeable membrane, however, presents problems should the membrane become structurally damaged, such as ruptured by the passage of a sharp object therethrough. The puncturing of membranes by abrasion or puncturing with sharp electrode components is particularly troublesome since it permits the cross migration of ions between the anode and cathode compartments. This reduces the efficiency of the cell. Frequently the cause of these abrading electrode components are roughened weld surfaces or the unrestrained electrode component that results when a weld on the electrode fails.
The number of welds on an electrode are numerous since each separately formed electrode element must be joined together. These elements include the individual electrode frame members, the electric current conducting elements and the electrode surfaces. The electrode surfaces, typically formed of a mesh, perforated or punched metal, especially have a large number of individually welded connections. Since the ion selective membranes are placed against these electrode surfaces, roughened weld surfaces or broken welds will easily puncture a membrane.
Welding presents the additional problem of building up heat in the electrode components as they are joined together in assembly, frequently causing warpage or distortion that affects the ability of the electrode to function efficiently. Warpage creates a non-planar surface that results in poor or non-uniform current distribution during operation across the electrode surface. This warpage also causes the welds to either fail completely or to be subjected to extreme stress at the weld points when subjected to the assembly pressures necessary to compress and assemble the cell in a configuration that requires planar electrodes.
These membranes are expensive. A commercial-size membrane cell, for example of the filter press type, will have up to twelve or more membranes in each electrolytic cell unit comprising multiple cathode and anode units separated by a membrane. Damaged membranes therefore either require expensive replacements or are time consuming to locate and repair. Consequently, repair or replacement is a costly manual process. The exact position of structurally damaged membranes in the electrolytic cell unit must be identified before they can be replaced.
Damaged membranes can also cause corrosion to occur in the metals used internally within the electrolytic cell by permitting the catholyte fluid with caustic or the anolyte fluid with brine to cross through the ruptured membrane and enter the adjacent electrode compartment. Caustic is extremely corrosive to the anode metals, while the brine with chlorine in the anolyte will dissolve nickel in a cathode, forming a high oxidation state nickel compound that will build up in the membrane. Where titanium is used in the anode, caustic will dissolve it, forming a titanium oxide that will also buildup in the membrane. This metallic ion buildup in the membrane decreases the current efficiency of the cell and increases the cell voltage.
Damaged membranes additionally will decrease the overall efficiency of the operating cell by decreasing the cathode and anode current efficiencies. The cathode current efficiency decreases are detectable in several ways, such as by measuring the weight of the caustic produced in a container vessel, calculating the actual production rate, and then comparing it to the expected production rate. Decreases in anode current efficiency are detectable because of the increase in the presence of oxygen in the cell gas and oxychlorides, such as hypochlorite, or chlorates in the spent anolyte stream, or spent brine, in a chloralkali cell. Changes in the pH of the spent anolyte stream are also indicative of such decreases. Obviously, damage to or ruptures of the membranes reduce the production capacity of the electrolytic unit, affect the quality of the cell product and adversely affect the economics of an operating commercial unit.
Another related disadvantage of electrodes which must be welded together at the joints between various components is the amount of labor involved. Welding, whether TIG welding, resistance welding or spot welding, is labor-intensive and requires sizeable periods of time. It is difficult to adhere to the required tolerances where hand operations such as welding, are performed. These tolerances are most critical in electrolytic cells that have no gap between the membranes and the adjacent cells. Additionally, the welds may not be consistently or uniformly made. The labor-intensive manual welding involved in the fabricating of an electrode significantly increases the cost of each cell. While it has been known for a number of years that certain metals, such as titanium and various alloys, may be metallurgically joined together by applying heat and pressure for a sufficient amount of time to cause intimate surface contact and interdiffusion of the atoms at the joint interface, such technology has not previously been successfully applied to electrolytic cell components. Similarly, while it has been known that certain metals, including titanium and various alloys, exhibit the characteristic of superplasticity or the ability of a material to develop unusually high tensile elongations under conditions of increased pressure and temperature, this process has not been previously successfully applied to the production of electrolytic cells. More recently, the combining of superplastic forming and diffusion bonding technology in making structural components has been accomplished, most notably in the aviation industry.
The cost and weight of the individual electrodes can be reduced if non-metallic materials can be used to fabricate the electrode frames, instead of the traditional metals of nickel, steel or titanium. Such material, for example plastic or ethylene-propylene diene monomer, hereafter referred to as EPDM, with or without polypropylene dispersed thereinto, must be protected from the corrosive effect of the fluids within the cell.
The foregoing problems are solved by the present invention wherein an electrolytic cell is fabricated from electrodes formed by the metallurgical bonding technique of diffusion bonding and the electrode frames are formed from a plastic or elastomeric material with all internal surfaces lined with a layer of protective material. The resulting cell has reduced electrical current flow resistance and a lower final cost.